A magnetoresistive device is used for an MRAM (magnetic random access memory) or a magnetic head sensor. The magnetoresistive device having a fundamental structure including a first ferromagnetic layer/an insulator layer/a second ferromagnetic layer is, utilizing the property such that the electric resistance is low when the direction of magnetization of the first ferromagnetic layer and the second ferromagnetic layer extend in parallel and is high when in non-parallel, adapted in such a manner that the direction of magnetization of one of the ferromagnetic layers is fixed and the direction of magnetization of the other one of the ferromagnetic layers is variable according to the external magnetic field to detect the direction of external magnetic field as a change in electric resistance. In order to obtain a high detection sensitiveness, high MR ratio (magnetoresistance ratio), which is an index of variation in electric resistance value between the cases in which the direction of magnetization is parallel and non-parallel is required. The inventor proposed a magnetoresistive device using magnesium oxide although referred to as MgO hereinafter, it does not mean that stoichiometry is 1:1) is formed into a film by sputtering as an insulator layer of the magnetoresistive device as a configuration in which a high MR ratio is obtained (for example, see Non-Patent Document 1 and Patent Document 1).    [Non-Patent Document 1] APPLIED PHYSICS LETTERS 86, 092502 (2005)    [Patent Document 1] Patent Application 2004-259280